In recent times, numerous proposals for employing active front-axle steering in motor vehicles have become known. Said proposals make possible a steering engagement at the front axle that is not dependent on the steering wheel angle. In the motor vehicles now available in series production, so-called superimposed steering systems are installed. In this special type of active steering, an angle is superimposed, that is, imposed, by means of a superimposition gearbox on a steering angle set by means of the steering wheel. The steering angle at the input of the steering box then no longer corresponds to the angle set by the steering wheel. Through this measure, a continuous adjustment of the steering ratio is possible on the one hand, so that, for example, for maneuvering at low speeds, it is possible to adjust a direct gear ratio, as a result of which, during parking, fewer turns of the steering wheel turns are needed, whereas, during highway driving, an indirect gear ratio can be adjusted, as a result of which the motor vehicle can be controlled with finer feel. On the other hand, the superimposition also permits fast engagements in terms of vehicle dynamics. This is understood to mean that, during driving, the wheel angle can be varied largely independently of steering engagements of the person steering the motor vehicle, thereby making possible a fast response in the regulation of vehicle dynamics.
Motor vehicles that include superimposed steering systems are subject to increased tolerance requirements in manufacture and assembly in regard to the steering system or have to be further limited in regard to permissible steering angles than is the case for motor vehicles without superimposed steering. These measures are aimed at preventing any undesired obstruction from being brought about during asymmetrical end stops, resulting under certain circumstances in a jerk at the steering wheel.
The direct consequence of a limited steering angle noticeable to the driver is that the maximum steering angle that exists in actuality cannot be fully exploited, this being manifested in a poorer handling during maneuvering, for example.
In conjunction with conventional servo steering without steering angle superimposition, it is known to record the steering end stops by way of steering movements. Thus, DE 10 2011 105 064 A1 describes a method for detecting and recognizing the end stops of a steering device of a motor vehicle, said steering device having a servo motor. In the process, the actual mechanical end stops of the steering device are recorded and, once the end stops have been recorded, software-controlled end stops are set. Further provided is a memory unit, to which a control device has access and in which steering parameters for different vehicle dynamics parameters and information on at least one permissible steering angle in each steering direction are saved. In a learning routine running with use of the control device, a steering force is applied manually to the steering wheel in both steering directions and the value thereof is recorded directly or indirectly. When the respectively applied steering force exceeds a predetermined value, the steering angle imposed at the input of the steering box is defined as an absolute maximum steering angle in the respective steering direction and saved in the memory unit.
It is further known from DE 10 2011 122 772 A1 to shift a steering device of a wheeled motor vehicle, in particular of a passenger car or truck, which has a steering drive, into a first direction and to record a first maximum travel distance in the first direction and to specify a first limit value on the basis of this recorded first maximum travel distance, and to limit electrically the travel distance of the steering device in the first direction on the basis of the first limit value, wherein the first maximum travel distance is recorded by the steering drive. The procedure in regard to the second (steering) direction is analogous.
Finally, DE 10 2012 022 900 A1 discloses a method and a device for adjusting a software end stop of a steering system of a motor vehicle during driving operation, wherein a steering apparatus makes possible an oversteering of the software end stop when predetermined driving conditions are met, wherein a value of any oversteering of the software end stop that has occurred is recorded by means of a steering angle sensor mechanism, a new position of the software end stop is determined on the basis of the recorded value by processor means, and the software end stop is adjusted to the new position by control means.
Known from DE 102 21 721 A1 is a motor vehicle with a steering system as well as a stop for limiting the maximum angle lock. In the process, it is possible to determine the value of the maximum angle lock as a function of driving parameters, such as, for example, the driving speed or the lateral and longitudinal acceleration. Common to the prior art is the fact that the procedures described are not suitable for superimposed steering.
Starting from the known prior art, the object of the invention is to provide a method that, in the case of motor vehicles with superimposed steering, enables permissible steering angles to be learned by way of manual steering engagements up to the mechanical or software-side stop, that is, to be defined and saved in memory. Furthermore, the object includes the provision of a motor vehicle that can implement the method in accordance with the invention.